Widespread FTTx/GPON technology has become one of the most interesting variant for Internet Service Providers in case of delivery Internet with other services like Video or IP telephony. Easy to implement, doesn’t require active components on the path from ISP to the client and very robust. What else makes FTTx/Gpon solution so interesting, how does it work and what appliances requires ?
Fiber to the X
For FTTx stands many types of architectures, which can be utilized. Each of them depends on chosen solution, distance from the Central Office to the the Client, possibilities and former used technology (usually on the customer site). If you glance at the picture below, you will notice, how far optical fiber reach in given FTTx technology. I focused on the most popular 3 out of them.
Choosen variant of FTTx forces on of the below topology, with all of them upsides and downsides. As you can see, besides passive technology we may choose active. But in this article I would like to focus on PON, especially on one of the variants, GPON namely.
GPON – Gigabit ethernet Passive Optical Network
The most interesting part is how FTTx technology is being provided, what utilizes and how works. GPON technology which is the most widespread in Europe within xPON technology, uses single mode fibers and WDM (Wavelength-division multiplexing) technology which enables in one single fiber provides two way traffic with use of different waves. GPON uses waves: 1310 nm in downstream (from OLT to the customer ) and 1490 nm upstream (from customer to the ISP OLT). Additionally may use 1550 nm wave in order to provide Video stream. The maximum speed of downstream is 2,488 Gb/s and upstream 1,244 Gb/s. In upstream direction TDMA (time division multiple access) mechanism is being utilized (each client gets his own slot (time window) during which may send). In downstream direction simple broadcast mechanism is being utilized, but the traffic is being encrypted with AES also for security. Maximum range is 20 km from OLT to a customer.
As you see, GPON technology consist of a few important devices:
ODN – Optical Distribution Network – optical network based on optical fibers, splitters.
OLT Optical Line Terminal – is a hub, which terminates optical fibers from optical splitters. OLT has got a role of central unit, which monitors and manages of ONT devices. Consists of many OLT ports and a few Gigabit Ethernet ports. Single OLT can manage and terminate even thousands of ONT terminals.
Optical splitter – passive component of PON, doesn’t require the power supply. Splits optical signal in relation from 1:2 up to 1:64. On the path between OLT and ONT may be placed a few optical splitters.
ONT/ONU – customer’s terminal, the end device on the path between OLT and customer. Usually fitted out in 1 SFP port for connection towards OLT and a few ethernet 100Mb ports towards the customer. May be fitted out in WiFi interface also.
Ok, we know what are both technologies, let’s try to calculate (in simplicity) link’s budget. We have to take into consideration below factors :
– total power for optical line (i.e for laser class C) =28 dB
– loss for optical fiber = 0,4 db/km
– loss on connectors and splicers (let’s assume) = 7 dB
– loss for optical spliter (let’s assume) 1:64 = 10*log(1in/64out) = -18 dB
So link’s budget in above example is : 28 dB – 18 dB – 7 dB = 3 dB
Maximum link, with split 1:64 from OLT to ONU is 7,5 km (3 dB/0.4 dB/km = 7.5 km)